Ink jet printers have become very popular due to their quiet and fast operation and their high print quality on plain paper. A variety of ink jet printing methods have been developed.
In one ink jet printing method, termed continuous jet printing, ink is delivered under pressure to nozzles in a print head to produce continuous jets of ink. Each jet is separated by vibration into a stream of droplets which are charged and electrostatically deflected, either to a printing medium or to a collection gutter for subsequent recirculation. U.S. Pat. No. 3,596,275 is illustrative of this method.
In another ink jet printing method, termed electrostatic pull printing, the ink in the printing nozzles is under zero pressure or low positive pressure and is electrostatically pulled into a stream of droplets. The droplets fly between two pairs of deflecting electrodes that are arranged to control the droplets' direction of flight and their deposition in desired positions on the printing medium. U.S. Pat. No. 3,060,429 is illustrative of this method.
A third class of methods, more popular than the foregoing, is known as drop-on-demand printing. In this technique, ink is held in the pen at below atmospheric pressure and is ejected by a drop generator, one drop at a time, on demand. Two , principal ejection mechanisms are used: thermal bubble and piezoelectric pressure wave. In the thermal bubble systems, a thin film resistor in the drop generator is heated and causes sudden vaporization of a small portion of the ink. The rapidly expanding ink vapor displaces ink from the nozzle causing drop ejection. U.S. Pat. No. 4,490,728 is exemplary of such thermal bubble drop-on-demand systems. In the piezoelectric pressure wave systems, a piezoelectric element is used to abruptly compress a volume of ink in the drop generator, thereby producing a pressure wave which causes ejection of a drop at the nozzle. U.S. Pat. No. 3,832,579 is exemplary of such piezoelectric pressure wave drop-on-demand systems.
The drop-on-demand techniques require that under quiescent conditions the pressure in the ink reservoir be below ambient so that ink is retained in the pen until it is to be ejected. The amount of this "underpressure" (or "partial vacuum") is critical. If the underpressure is too small, or if the reservoir pressure is positive, ink tends to escape through the drop generators. If the underpressure is too large, air may be sucked in through the drop generators under quiescent conditions. (Air is not normally sucked in through the drop generators because the drop generators comprise capillary tubes which are able to draw ink against the partial vacuum of the reservoir.)
The underpressure required in drop-on-demand systems can be obtained in a variety of ways. In one system, the underpressure is obtained gravitationally by lowering the ink reservoir so that the surface of the ink is slightly below the level of the nozzles. However, such positioning of the ink reservoir is not always easily achieved and places severe constraints on print head design. Exemplary of this gravitational underpressure technique is U.S. Pat. No. 3,452,361.
Alternative techniques for achieving the required underpressure are shown in U.S. Pat. No. 4,509,062 and in copending application Ser. No. 07/115,013 filed Oct. 28, 1987, both assigned to the present assignee. In the former patent, the underpressure is achieved by using a bladder type ink reservoir which progressively collapses as ink is drawn therefrom. The restorative force of the flexible bladder keeps the pressure of the ink in the reservoir slightly below ambient. In the system disclosed in the latter patent application, the underpressure is achieved by using a capillary reservoir vent tube that is immersed in ink in the ink reservoir at one end and coupled to an overflow catchbasin open to atmospheric pressure at the other. The capillary attraction of ink away from the reservoir induces a slightly negative pressure in the reservoir. This underpressure increases as ink is ejected from the reservoir. When the underpressure reaches a threshold value, it draws a small volume of air in through the capillary tube and into the reservoir, thereby preventing the underpressure from exceeding the threshold value.
While the foregoing two approaches for maintaining reservoir underpressure have proven highly satisfactory and unique in many respects, they nonetheless have certain drawbacks. For example, in the pen described in the above-referenced patent, as the flexible bladder reaches its fully collapsed state, the underpressure increases to the point that the drop generator can no longer draw ink therefrom and printing ceases with unused ink left in the bladder. The pen described in the above-referenced application is limited in the temperature and altitude extremes to which it can function properly. For example, if such a pen is transported in an aircraft cabin that is pressurized to an 8000 foot elevation, any air in the ink reservoir will expand in volume by a factor of approximately one third. If the volume of air in the reservoir is more than three times the volume of the catchbasin to which overflow from the capillary reservoir vent tube is routed, the air's expansion will drive more ink into the catchbasin than it can contain and the catchbasin will overflow. This problem can be solved by making the catchbasin large enough to contain the ink in any possible altitude or temperature circumstance, for example, by making the size of the catchbasin fully 35 percent the size of the ink reservoir.
Even this solution, however, is not wholly satisfactory. The only means of returning ink from the catchbasin to the reservoir is a drop tube extending therebetween. In most orientations, the ink in the catchbasin cannot all be returned to the reservoir as the reservoir's air volume shrinks because the catchbasin's ink level will fall below the end of the drop tube. In such case, air will be sucked into the reservoir through the exposed end of the tube. It can be appreciated that a second expansion of the air in the reservoir (due to increasing altitude or temperature) will pump more ink into the catchbasin than previously since the volume of air in the reservoir has increased. Furthermore, the catchbasin already contains some ink from the previous altitude or temperature cycle. Consequently, the second altitude or temperature excursion may fill the catchbasin beyond its capacity, forcing ink out the catchbasin's vent tube and out of the pen, a very undesirable outcome. Each subsequent environmental cycle exacerbates the problem.
It is an object of the present invention to provide an ink jet pen that overcomes this problem.
It is a more particular object of the present invention to provide an ink jet pen that can undergo repeated altitude and temperature excursions without leaking ink.
According to one embodiment of the present invention, an ink jet pen is provided with a plurality of ink pickup tubes that extend from the pen's ink reservoir down into different regions of its catchbasin. The pickup tubes are arranged so that, if there is any ink left in the catchbasin, one of the tubes will be located so that its end is immersed in the ink, permitting the tube to draw the ink back into the reservoir, regardless of the pen's orientation.